Gas turbine engines, particularly those used to propel aircraft, are subject to non-uniform heating and cooling during transient operating conditions. Such conditions occur, for example, during aircraft takeoffs and landings. During an engine transient, various engine parts and structures experience differential thermal growth rates; in other words, they expand and contract at different rates. Thus, when an engine is throttled back, during an approach for a landing, for example, the engine casing, which is a thin metallic structure, will typically cool faster than the inner, more massive, rotating engine parts. The casing will contract inwardly and thereby create a potential for rubbing contact between various static and moving engine parts.
Rubbing contact between engine parts is generally undesirable for at least two reasons. First, the engine parts can be damaged, which can result in replacement costs. Second, the rubbing creates gaps between engine parts that allow an unwanted efficiency wasting flow of gas to occur between the engine parts. To inhibit engine damaging rubbing between parts, engines are designed with clearance gaps of a predetermined size between stationary and moving engine components. These clearance gaps would allow air or gas to flow from one portion of the engine to the other, resulting in a loss of engine efficiency as just described, unless appropriate seals are used to restrict such flows.
In a typical gas turbine engine, labyrinth seals are disposed between rotating and stationary members to restrict the flow of air or gas between upstream high pressure regions and downstream low pressure regions of the engine. A labyrinth seal usually consists of one or more pointed hardened seal teeth disposed on a rotating, substantially cylindrical, member and running in close proximity to a cylindrical or stepped cylindrical stator or stationary member typically carried in some manner by the engine casing. The seal teeth act to restrict the flow of air or gas between the two members from the upstream to the downstream region. On some gas turbine engine applications, particularly inter-shaft seals, the cylindrical "stator" just referred to also rotates independently of the toothed seal member. A labyrinth seal restricts the air flow one or more times depending upon the number of teeth used between the two seal members. The total pressure drop across the seal is the sum of the individual pressure drops experienced across each tooth of the seal.
While labyrinth seals generally perform well when new, they are subject to a loss of efficiency over time because of the rubbing contact referred to above. More specifically, during an engine transient of the type where the engine slows, the engine casing, which has a lower thermal capacity, is subject to thermal contraction due to cooling at a quicker rate than the rotor portion, which has a higher thermal capacity relative to the engine casing, carrying the seal teeth. As the casing carrying the stationary seal member contracts, the individual seal teeth tend to dig into the seal lining mounted on the seal land, i.e., the seal member mounted on the stator member of the seal. This may create grooves in the seal and lining and when the engine is again operated at cruise or accelerated conditions, that is, when the engine casing has expanded to operational size, a gap may now be present between the teeth and the lining that allows additional, unwanted air flow through the seal.
Over several thousand hours of operation a labyrinth seal will become progressively less able to accomplish its sealing function effectively. To be effective over the long term, clearance control between the rotating and stationary labyrinth seal members during engine transients is of critical importance and needs improvement. While such clearance control is theoretically possible, the weight and cost of the apparatus necessary to achieve the control make it impractical. It would be desirable, therefore, to provide a seal more tolerant of thermal transient conditions and less subject to efficiency losses.
One type of seal that meets the just mentioned criteria is a brush seal. A brush seal typically consists of a plurality of bristles disposed between a facing plate and a backing plate. The seal is most often attached to a stationary seal member, and the ends of the bristles, which project beyond the backing plate, make contact with a rotating seal member. Because the bristles can flex whereas the hardened seal teeth of the labyrinth seal cannot, a brush seal is more tolerant of changes in the size of the engine dimensions during engine transients. That is, in an application such as a gas turbine engine, when the engine casing contracts more quickly than the rotating parts, thus narrowing the clearance gap between them, the bristles can "take up" the reduced gap size by flexing radially whereas labyrinth seal teeth would gouge the lining.
The effectiveness of brush seals are, however, limited in other ways. For example, the very bristle pliancy that enables them to tolerate thermal excursions better than a labyrinth seal limits the pressure drop that the seal can tolerate. Thus, where the pressure drop across a seal is too great for a single set of bristles to handle, multiple stages are used. It is known, however, that the pressure drop across each stage increases from stage to stage from the high pressure region to the low pressure region. Even though multiple stages of bristles are used in the seal, it is still possible that the pressure drop across the last stage of the seal will exceed the capabilities of that stage, and that the end result may be seal failure.
One apparatus for addressing this problem is shown in U.S. Pat. No. 4,756,536 to Belcher. In this particular application of a brush seal it is proposed to decrease the pressure drop across the final stage of the seal by diverting air from the high pressure side of the final stage into a third pressure region and in some applications, from the third pressure region into the downstream side of the seal. This is accomplished through the use of an apertured spacer ring disposed upstream of the seal stage facing plate. Air or gas passes through the spacer ring apertures and then vents through a plurality of apertures located in the static structure to which the seal stages are attached. While perhaps serving to accomplish the goal of reducing the pressure drop across the final stage, this particular method requires an engine geometry that allows air to be bypassed into a third pressure area of the engine. This design may make the engine heavier and, due to the machining costs of machining apertures in the spacing ring and in the static engine structure, is sure to increase the cost of the engine. In addition, placement of the apertures in the static structure is sure to entail additional manufacturing problems. Finally, a further disadvantage of this particular method is that the apertures are subject to blockage. If the apertures become blocked they will not vent properly and, as a result, the final stage may fail because the pressure drop will increase across that stage to a level that may exceed its capabilities.